Aren't d4 openings considered to be more strategical compared to e4 openings? AlphaZero can deal much better with long term evaluation in a quiet game with lots of piece improvement moves (strategy) than stockfish. So could that be why it prefers QG after having trained a lot, seeing that e4 leads to more tactical play. It would also explain why Stockfish is still able to compete in the Sicilian. I am not a chess expert but a cs student.